The effects of acute normovolemic hemodilution and autologous blood transfusion were studied in open heart patients, compared with rather healthy patients, NYHA class 1-2 and the high risk patients, NYHA class >2. Thirty-nine patients were involved in this study, 15 of them were identified as the rather healthy group while 24 patients belonged to the high risk group. Acute hemodilution was performed after anesthetic induction and before heparinization. Using an equal volume of polygeline 3.5 per cent (Haemaccel) to replace autologous blood removal, the number of patients who needed inotropic support to achieve normal arterial blood pressure was not significantly different between the groups. Following retransfusion of pump perfusate and autologous blood after the termination cardiopulmonary bypass, the number of patients who received additional homologous blood as well as the amount used percase were significantly less in the rather healthy patients. There was none in this group, but half of the high risk patients suffered from serious perioperative complications and one died. We conclude that this technique is safe and benefits blood conservation in rather healthy cardiac patients undergoing open heart surgery, but special precautions against risk should be considered in high risk patients.